New 7D user
just a quick question. i recently purchased a Canon 7D with a Canon 70-200mm f/4 IS lens. for the most part, it's an awesome camera, however, when i use the zoom lens to take pics at my sons basketball game they come out grainy. i've tried playing around with the settings but i just cant seem to get a really crisp shot. could it be that the f/4 is not good enough and i would need to upgrade to a f/2.8? any help would be great.